It is conventionally known to use a DC brushless motor as a drive power source of an electric automobile. In order to allow the electric automobile to effect braking similar to the “engine brake” of an automobile with an internal combustion engine when the automobile travels a long downslope, for example, it has been proposed to conduct a regenerative control of the motor (see Japanese Patent Application Publication No. 2002-58277).
As the automobile gains speed while traveling a long downhill with the drive control being suspended, the motor rotation speed can exceed a no-load rotation speed of the motor, and in such a case, a regenerative current flows to the power supply (battery) from the motor via a control device (controller). However, in the conventional drive apparatus for a DC brushless motor, such a regenerative current cannot be controlled, and this may cause an overcharge of the battery or an unfavorable increase of the motor friction due to an excessive increase in the regenerative energy.
In the above publication No. 2002-58277, the motor operation is divided into four mode (or regions) according to the torque and rotation speed, where the four regions comprise a normal field powering control region, a normal field regenerative region, a weak field powering control region, and a weak field regenerative region. For example, the motor may be initially operated in a powering mode with a weak field, and as the rotation speed increases, the motor may be operated in a no-load drive mode (zero torque) and then in the regenerative mode with a weak field.
As a way for reducing the field (or field current), it is known to short-circuit the field coil, insert a resister in the circuit, chopper-control the field current, or conduct field added excitation control, but they are still not satisfactory in achieving both of simple control structure and highly accurate control.